Water Rat
Hydromys chrysogaster
DESCRIPTION AND CHARACTERISTICS
Life form / morphology
The water rat is a distinctive native rodent,
specialised for an aquatic life. It is a
relatively large mammal with broad,
partially-webbed hind feet and water
repellent fur, making it perfect for an
aquatic existence. Its body is streamlined
for swimming, with abundant whiskers and
small eyes and ears. They can reach up to
40cm in length, excluding the tail.
Colouration varies from slate grey to
nearly black on the back, and white to
orange on the underside. The tail is thick
and usually has a white tip, an identifying
feature of this interesting critter.
or foraging during the daylight hours of
early morning or early evening.
Surprisingly, the Water Rat is not well
adapted for cold or hot climates, not being
able to maintain its body temperature
below 15° and risking hyperthermia at
temperatures above 35°. Unlike the
Platypus, they do not have very insulating
Feeding habits and behaviour
The water rat is mainly carnivorous;
crustaceans, aquatic insects and fish form
the bulk of its diet. Small birds, eggs,
mammals, frogs, reptiles and plants may
also be eaten. They hunt partly by sight
when diving, and by stalking through
vegetation along the waters edge.
Typically they forage close to the waters
edge, catching prey in the mouth and
often taking it to a favourite feeding place,
where remains are often left.
The water rat is mainly nocturnal; being
active mostly in the hours following
sunset, but may also be found swimming
Photo: Tony Robinson/Viridans Images
fur, so they tend to remain in the burrow
during the heat of the day, and on winter
nights foraging is interspersed with
warming in the burrow. In winter they also
rely less on aquatic prey and more on
terrestrial prey, minimising time spent in
the water. These adaptations have
allowed the water rat to become one of
Australia’s most successful & widespread
rodents.
LIFE CYCLE AND MATING
Water rats mate in late winter and spring. The
gestation period is 34 days, with litters born from
September to February. Litter size varies from
one to seven, with the average litter size being
four to five. The female water rat will have one to
five litters per breeding season. A nest is
constructed in a burrow close to the water,
where the young are born and suckled for
around four weeks. It is believed that sexual
maturity in the wild is not reached until over 12
months of age, and females will breed until 31/2
years old. Water rats are solitary excluding when
mating or the female has dependant young.
Water rats are territorial and may be aggressive
when populations are at a high density. Fighting
is common among adults, with both sexes often
having bitten or scarred tails.
HABITAT AND DISTRIBUTION
The water rat is found in Australia, New Guinea
and a number of adjacent islands. It is widely
distributed in eastern Australia, occurring west to
the Kimberley's along the northern coast and
west to Eyre Peninsula on the south coast. It is
widespread throughout Victoria, found in fresh,
brackish and salt water bodies, including shallow
lakes, deep water storages, farm dams, slow
moving rivers, creeks, estuaries, coastal lakes
and sheltered marine bays. They prefer low
banks to flat water edges, with many sites
having dense vegetation cover on the banks.
Photo: Andrew McCutcheon/rakali.com
IMPORTANCE AND POTENTIAL THREATS
Water rats were not protected by legislation until
1938. Prior to this date, they were widely hunted
for their fur, but populations now appear to have
recovered.
Although water use practises would have surely
removed much of its habitat, the water rat is
now of least conservation concern and its
current range is likely to be similar to those
areas occupied prior to European Settlement in
Australia.
Traditionally, the water rat has been considered
to be a pest in the irrigation districts, with belief
that its burrows in channel banks and other
irrigation structures cause leaking and collapse
of structures. Some now regard this damage as
less than that caused by freshwater crayfish,
whose numbers may well be controlled by the
water rat.
Predators of the water rat include birds of prey,
cats, snakes and large fish (mainly the young
are vulnerable).
They also appear to have the unusual ability of
being able to kill the cane toad without being
poisoned.
References:
Menkhorst, P. W (ed), 1996, Mammals of Victoria: Distribution, Ecology & conservation, Oxford University Press, Melbourne.
CSIRO, Water for a Healthy Country, 2004, Taxon Attribute Profiles, Hydromys chrysogaster, available on line at:
http://www.anbg.gov.au/cpbr/WfHC/Hydromys-chrysogaster/index.html